Last Updated: Nov 30, 2021     Views: 195

Image: A mahogany and inlaid optical diagonal machine or zograscope, with mirrored pane and magnified viewer on height adjustable stem and turned, weighted, circular base. 2019.8.15.

From Dedo von Kerssenbrock-Krosigk, Curator of European Glass:

Thank you for your interesting enquiry. I confess that I had to look up what exactly a zograscope is, and came across Blake, Erin C. “Topographical Prints Through the Zograscope.” Imago Mundi 54 (2002): 120–24. This article refers to an contribution to the Gentleman’s Magazine v. 19, December 1749, pp. 534–535, with a nice woodcut representation of a zograscope of the time.

Since I am not a scientist, I forward your enquiry to Robert H. Brill, research scientist at our museum. He may have some corrections to my own answer, and maybe he can direct you to an institution that can give you more substantial information on the history of optical glass.

I expect, however, that there is no easy answer. The quality of optical glass significantly augmented at the turn of the 18th century. However, until way into the 20th century (and perhaps, to some degree, even today) a considerable amount of luck was involved in the production of optical glass. The right composition of the glass batch is a crucial, but not the only factor in making good optical glass. Most importantly in the past, the chunks of glass that were most suited to be cut into lenses had to be hand-picked from a large cast of "optical," yet inhomogeneous glass. Therefore, I would not be surprised if a few chosen glass lenses of the mid-17th century proved to be better than a sub-standard optical glass quality of about a century later.

However, I would not even expect that the zograscopes were fitted with lenses of the best available quality. These were apparently no scientific instruments which required high quality, such as telescopes and microscopes. Instead, it seems that the distortions of the optical apparatus were in fact required to obtain the three-dimensional sensation (cf. Blake, p. 120). All in all, I do not think that a zograscope of the early 19th century necessarily had a better optical quality than one of the mid-18th century.

It is certainly possible to reproduce an optical glass batch of the 18th century, if a detailed-enough recipe is available. Using modern equipment, you would probably come up with the best quality of optical glass that could have been gained from such a batch (as I mentioned, the quality was certainly not homogeneous). However, there were no standard specifications for 18th century glass. In reproducing one recipe, you will gain glass from one factory at one given date, and it will probably remain completely open (and rather doubtful) if this glass ever made its way into a contemporaneous zograscope.

In short: I don't think that there is much sense in going into the effort of reproducing glass of a given time to distinguish the optical subtleties when used in a zograscope. The changes of the overall design of these devices may have had a much more dramatic effect.


From Dr. Robert Brill, Research Scientist Emeritus:

I have read through your message and Dedo's reply.  It seems to me that his reply is entirely accurate (as I would expect it to be) -- and I have little to add to it.  In particular, I don't believe there would be much point to trying to reproduce a lens having the exact glass composition of the times.  There are too many other variables involved.  I think the logical next step would be to examine your prints through whatever instrument you happen to have at hand regardless of date.  But I imagine you have already tried that.

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